In a blast furnace method which is the most common method for pig iron production, coke plays a number of roles, for example as a reducing agent for iron ore, as a heat source and as a spacer. In order to operate a blast furnace stably and efficiently, it is important that the gas permeability in the blast furnace be maintained. Thus, there has been a need for high-strength coke to be produced. Coke is produced by carbonization of a coal blend, which is a blend of various types of coking coals that have been crushed, in a coke oven. During carbonization, coking coal softens and melts at temperatures in the range of about 300° C. to 550° C. and, at the same time, volatile matters are driven off to form a gas which causes swelling, whereby particles are adhered together to give a mass of semicoke. The semicoke is thereafter densified by being contracted in the course where the temperature is raised to near 1000° C., resulting in a rigid coke (a coke cake). Thus, the adhesiveness of thermally plastic coal greatly influences properties such as coke strength and particle diameter after carbonization.
In order to enhance the adhesion of coking coal (coal blend), a coke producing method is generally adopted in which a coal blend is mixed with a caking additive that exhibits high fluidity at temperatures where the coal becomes softened and molten. Here, examples of the caking additives include tar pitches, petroleum pitches, solvent-refined coals and solvent-extracted coals. Similarly to coal, the adhesiveness of these caking additives in a thermally plastic state greatly affects coke properties after carbonization.
In the production of coke in a coke oven, carbonized coke is discharged from the coke oven with a pushing machine. If the degree of shrinkage of the produced coke cake itself is low, discharging out of the oven becomes difficult. This can lead to a “stickers (or hard push)”, namely, a problem in which the coke cannot be discharged from the oven. The structure of a carbonized coke cake is largely affected by volume changes of coal and semicoke during the carbonization process. It is known that the shrinkage of semicoke has a good correlation with the volatile content of coal (see, for example, Non Patent Literature 1). In many cases, the volatile contents of coal blends are controlled to be substantially constant for operations in the same plant. Thus, volume-change characteristics of plastic coal greatly affect the structure of a carbonized coke cake.
As mentioned above, thermal plasticity of coal is very important due to their great influences on coke properties and coke cake structures after carbonization. Thus, methods for measuring these characteristics have been studied actively since old times. In particular, coke strength, which is an important coke quality, is largely affected by properties of raw-material coal, especially coal rank and thermal plasticity. Thermal plasticity is exhibited when coal becomes softened and molten when heated, and are usually measured and evaluated with respect to properties such as fluidity, viscosity, adhesiveness and swellability of thermally plastic coal.
Of the thermal plasticity of coal, the fluidity of thermally plastic coal is commonly measured by a coal fluidity testing method based on a Gieseler plastometer method specified in JIS M 8801. According to a Gieseler plastometer method, coal that has been crushed to sizes of not more than 425 μm is placed in a prescribed crucible and is heated at a specified temperature increase rate while the rotational speed of a stirring rod under a specified torque is read on a dial and is indicated in terms of ddpm (dial division per minute).
While a Gieseler plastometer method measures the rotational speed of a stirring rod under a constant torque, other methods evaluate the torque at a constant rotational speed. For example, Patent Literature 1 describes a method in which the torque is measured while rotating a rotor at a constant rotational speed.
Aimed at measuring viscosity that is a physically significant thermal plasticity, there are methods for measuring viscosity with a dynamic viscoelastometer (see, for example, Patent Literature 2). Dynamic viscoelastometry is a measurement of viscoelastic behaviors observed when a viscoelastic body is subjected to periodic forces. In the method described in Patent Literature 2, the viscosity of thermally plastic coal is evaluated based on complex viscosity coefficient among parameters obtained by the measurement. This method is characterized in that the viscosity of thermally plastic coal is measurable at a given shear rate.
Further, it has been reported that thermal plasticity of coal is evaluated by measuring the adhesion of thermally plastic coal with respect to activated carbon or glass beads. In such a method, a small amount of a coal sample, sandwiched vertically between activated carbons or glass beads, is heated to thermal plasticity and is thereafter cooled, and the adhesion of the coal with respect to the activated carbons or the glass beads is visually observed.
A common method for measuring the swellability of thermally plastic coal is a dilatometer method specified in JIS M 8801. In a dilatometer method, coal that has been crushed to sizes of not more than 250 μm is compacted by a specified method, placed into a prescribed crucible and heated at a specified temperature increase rate while the displacement of the coal is measured over time using a detection rod arranged on the top of the coal.
In order to simulate thermally plastic behaviors of coal in a coke oven, coal swellability testing methods are known which achieve enhanced simulation of permeation behaviors for a gas generated during the plasticization of coal (see, for example, Patent Literature 3). According to such a method, a permeable material is arranged between a coal layer and a piston or is arranged between a coal layer and a piston as well as at the bottom of the coal layer so as to increase pathways through which volatile matters and liquid substances generated from the coal can pass, thereby approximating the measurement environment more closely to an environment in which swelling behaviors actually occur in a coke oven. A similar method is also known in which the swellability of coal is measured by arranging a material having a through pathway onto a coal layer and microwave-heating the coal while applying a load thereto (see Patent Literature 4).